An apparatus including a plasma process chamber is described in U.S. Pat. No. 6,074,488, which is incorporated herein by reference. This apparatus comprises a process chamber with a support assembly. The support assembly contains a dielectric member as a support element that is adapted to carry a substrate during processing. The support assembly further comprises a collar that encircles the support element. A voltage with a radio frequency (RF) is applied to the collar in order to influence the plasma distribution near a peripheral portion of the support element. In this way, the processing of the substrate is modified.